Fix filetypes in new scratch buffers

new_buffer was using incorrect indexing for the filetype argument and
always used no filetype. Add documentation of what the optional argument
is for and use it.

Fixes #4.
Add Plug-mappings

Fixes #3: Add plug mappings for invoking itchy.
itchy-open-same-filetype-scratch is the most interesting, but for
completeness I'm exposing itchy-open-scratch too.